Matter Transmission

Abstract

In a world connected by wifi we take the wireless transmission of information for granted. But can matter be similarly transmitted? The matter transmitter described in Milne’s story would of course not work as advertised, but physicists are working on a quantum version of a matter transmitter—a quantum teleporter. Quantum teleportation isn’t quite the equivalent of Milne’s device, or Star Trek’s more famous Transporter, but the technology is sure to change our world in the coming decades.

Supplementary material

Commentary

Most stories in this book require modern readers to suspend their knowledge of science and technology. This is particularly the case with “Prof. Vehr’s electrical experiment”. The author, Robert Duncan Milne, has a character suggest there’s nothing extraordinary about the transmission of information in the absence of wires to carry the information: after all, “clairvoyants and trance mediums” seem to do this all the time. (He was not alone in his blithe acceptance of clairvoyance. Arthur Conan Doyle may have been a more famous writer than Milne but he was also more gullible: Doyle attended séances and made countless public speeches to promote his belief in ghosts, mediums, and the spirit world.) For modern readers, of course, the wireless transmission of information is entirely unremarkable—our daily lives depend upon it—but the notion that clairvoyance is a routine phenomenon, merely a form of mental WiFi, strikes us as foolishness. So how could Milne get it so wrong?

Milne was writing in 1885. For his readers, the transmission of information over wire was well established. The first electrical telegraph systems had been developed in the 1830s, and by the 1870s many parts of the world had been linked by cable. Science fiction writers are in the business of extrapolation, so it was natural to want to describe a technology enabling long-distance communication without wires. The easiest way to frame such a technology was by invoking clairvoyance—Milne’s readers would certainly have encountered tales of “clairvoyants and trance mediums” and, even if they dismissed such ideas as nonsense, they would have understood what Milne was getting at. Had Milne sought to frame wireless communication using the physics available to him then his readers would undoubtedly have been confused.

The notion that electromagnetic waves can propagate freely through space—and thus permit wireless transmission of information—was developed mathematically by James Clerk Maxwell in 1864. Few if any of Milne’s readers would have been able to appreciate the implications of Maxwell’s work, however, and the existence of radio waves was demonstrated experimentally only after the publication of Milne’s story. It wasn’t until 1886 that Heinrich Hertz began a series of experiments that proved the reality of electromagnetic waves; and radio waves would not form the basis of a commercial wireless telegraphy system until the turn of the century, when Marconi began investigating the technology. The first practical television systems, which could transmit live moving images, were not developed until the 1920s—four decades or so after Milne’s story. So we can reasonably cut Milne some slack in this regard: what we take for granted is a relatively modern advance.

In his story, however, Milne goes far beyond what our radio and television technology permits us to do. Clairvoyancy is supposed to permit not only the wireless transmission of information but also the wireless transmission of matter. And matter transmission, or teleportation, is impossible—isn’t it?

Teleportation is a science fiction staple. The most famous example of a matter transmitter is surely Star Trek’s Transporter—a wonderful plot device that meant Captain Kirk had only to utter the phrase “Beam me up, Scotty” before being whisked to safety—but matter transmission formed the basis of numerous SF stories. Although writers in the Golden Age of science fiction might have doubted the possibility of a practical teleportation system, they were in a better position than Milne to provide a rationale for the phenomenon. Radio and TV sets relied upon the wireless transmission of information, so SF readers could accept the following hand-waving explanation of matter transmission. A matter transmitter would “scan” an object (to obtain full information about the material of which it was made), transmit that information to a receiving station, and then use the information to reconstitute the object. Simple, no?

No one suggested it would be easy to make a matter transmitter but the recipe outlined above—“scan” an object at point A; transmit full information about the object to point B; use that information to reconstitute the object—sounds as if it should work in principle. Put this way, teleportation sounds like an extremely advanced engineering problem. But then physicists took a look, and pointed out that quantum mechanics spoils this wonderful science fictional dream of matter transmission.

The difficulty is the Uncertainty Principle. This fundamental tenet of quantum mechanics says you can’t simultaneously measure the position and momentum of a particle. But that means you can’t get full information about an object when you “scan” it. This lack of complete information ensures the object recreated at point B can’t be a completely faithful copy of the original. In the worst case, the recreated object might end up being undifferentiated mush—a prospect that worried Star Trek’s Dr McCoy whenever the medic was forced to use the Transporter.

So physicists had shown how a fundamental principle of quantum mechanics ruled out a working teleportation device. “Shame”, I hear SF readers say. But then, in 1993, Charles Bennett and colleagues showed how a different fundamental principle of quantum mechanics does indeed permit a form of teleportation! So—what’s going on?

The key to quantum teleportation is the mysterious phenomenon of quantum entanglement.

Entanglement occurs when a pair of particles interact in such a way that the quantum state of one particle can’t be described independently of the other—even if the particles are widely separated. (Figure 10.1 is an artist’s attempt to represent a concept that is perhaps incapable of being represented.) Entangled particles possess properties—spin, polarization, and so on—that are correlated. For example, suppose we create a pair of entangled particles with a total spin of zero. Then, if we measure the spin of one particle to be “up” then the spin of the other particle will be measured to be “down”—no matter how widely separated the two particles happen to be. The mysterious aspect of this phenomenon arises because, until a measurement is made, each of the quantum particles will be in a superposition of spin-up and spin-down states. To put it crudely, the particle hasn’t decided whether it’s spinning up or down until we observe it. Making a measurement here causes the state of the particle here to be spin-up (or spin-down, depending on what we observe); but the measurement instantaneously causes the state of the other particle over there to be spin-down (or spin-up, depending on what we observe here). And that instantaneous effect happens regardless of the size of the separation. The particles could be on different continents or on different planets—entanglement still holds.

An artist’s attempt to represent a phenomenon which, although it has been demonstrated in the laboratory, is almost impossible to visualise. The goal of quantum teleportation is to transfer the quantum state of particle A onto a distant particle B—which is possible if the particles are entangled. When the goal is achieved it’s information, not matter, that’s transmitted. In everyday language, however, the net effect is the same as transmitting a quantum particle from A to B. An artist created this image to illustrate an experiment, carried out in 2009, that teleported an atomic state over a distance of one metre, or about 3 feet: the state of ion A on the left got scrambled but was reconstructed at ion B on the right (Credit: Nicolle Rager Fuller, NSF)

Entanglement seems paradoxical but experiments demonstrate its validity. Entanglement may be a mystery but we may as well learn to live with it. How, though, does entanglement get round the Uncertainty Principle and help with teleportation?

Let’s keep things simple, and agree to teleport a single particle. Alice is going to send the particle and Bob is going to receive it.

Alice starts by entangling two particles—let’s call them A and B—and she keeps particle A while sending particle B to Bob. This sets up the transmission channel: if Alice performs some operation on particle A then instantaneously this affects the state of particle B. Now, suppose Alice has a particle C whose quantum state she wants to teleport to Bob. To do this she makes a particular type of quantum measurement on particles A and C. The act of measuring erases the state from C: the particle C no longer exists in the original form. However, the entanglement between A and B means B is in a state that can be turned into whatever state C had—so long as Bob makes the correct operation on B. In order for Bob to know what operation to apply, Alice has to send him the outcome of her measurement—and she has to do this by a classical channel (she could phone him, send him a postcard, email him, or whatever). Once Bob has this information he can turn B into a state that’s identical to C. And because the state of a quantum particle tells you all there is to know about the particle—all that it’s possible to know about the particle—Alice has succeeded in teleporting particle C to Bob.

It’s important to understand that Alice can’t use this recipe to transport matter or energy; what gets transferred is a quantum state. Furthermore, Alice can’t use this recipe to transmit information faster than light: entanglement might be instantaneous, but in order for teleportation to work Alice must also transmit classical information, which can only happen at light speed or slower. Nevertheless, Alice can use the recipe to teleport a quantum state. The recipe might seem complicated, but physicists have demonstrated it successfully in the laboratory.

In 1998, just five years after publication of the initial protocol outlining how quantum teleportation might work, physicists succeeded in teleporting a single photon over a small distance. Six years after that, they succeeded in teleporting a photon over a distance of 600 m. Then they learned how to teleport electrons, atoms, and superconducting circuits. At the time of writing, the record distance for teleporting a quantum system is 1400 km. So quantum teleportation is now well established. Quantum teleportation couldn’t explain the events in Milne’s story, nor can it be used to construct a Star Trek Transporter. But it will form the basis of usable technologies—just not the technologies SF writers once dreamed of.

Clairvoyance also plays a role in the next story. When it came to séance-based mumbo-jumbo Fitz-James O’Brien, the next story’s author, was as credulous as Milne and Doyle. It’s a shame these authors were unaware of quantum theory—as we’ll see in Chapter 11, this theory gives us our best understanding of the world on the smallest scales.

Notes and Further Reading

Doyle attended séances—Doyle has been the subject of numerous biographies, some of which focus on his life beyond the creation of Sherlock Holmes and Professor Challenger. See, for example, Stashower (1999); Polidoro (2001); Miller (2008); Fox (2018).

wireless transmission of information—Lewis (1991) is a good account of the development of radio, with a particular emphasis on American pioneers; Burns (1998) gives the early history of television.

Star Trek’s Transporter—For more details of the Transporter, along with photon torpedoes, visors, and more, see Siegel (2017).

Alice starts by entangling two particles—Entanglement will provide the basis of several new technologies, including quantum computing. Aaronson (2013) discusses the deep principles behind these emerging technologies. Aaronson’s book is funny, too.